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Insulation Calculator

Pick blown-in or batts. Blown-in uses Owens Corning's published AttiCat coverage chart for the R-value you want; batts use the coverage printed on the package, exactly as the label rates it.

Diagram of a wall showing its length and height, with a door and a window
One wall at a time: length × height is the area every wall material covers.

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How insulation quantities are calculated

Blown-in (attics): manufacturers rate blown insulation by bags per 1,000 sq ft for each R-value. Installing fewer bags than the chart means a lower R-value than the one on the bag. This tool encodes Owens Corning's AttiCat fact-sheet chart for its 28.5 lb bag: R-30 takes 15.3 bags per 1,000 sq ft (10.25 in settled), R-38 takes 19.5 (13 in), R-49 takes 25.8 (16.5 in). Bags = your area ÷ 1,000 × the chart rate, rounded up. Other brands' bags differ. If you're not using AttiCat, check your bag's own chart.

Batts: a batt package's coverage is printed on its label as gross square feet of product: packages = area × (1 + waste) ÷ sq ft per package, the standard way the label figure is used. Batt widths are made to fit the framing: 15 in wide batts fit 16 in on-center studs; 23 in batts fit 24 in on-center. Buy the width that matches your framing, and the R-value your climate zone calls for.

Attic areas are just length × width of the ceiling below. For walls, use the wall's length × height. Fiberglass in an existing attic can usually be blown right over. The chart rates the added R-value the same way. Hanging drywall over it afterwards? The Drywall Calculator counts sheets, mud and tape for the same walls.

Sources

The figures behind this calculator come from published manufacturer data and industry standards:

Worked examples

1,000 sq ft attic to R-38, blown-in

AttiCat chart: 19.5 bags per 1,000 sq ft at 13 in settled depth.

Buy 20 bags

1,200 sq ft attic to R-30, blown-in

15.3 bags per 1,000 sq ft × 1.2 = 18.4 bags.

Buy 19 bags

500 sq ft of walls, batts at 90 sq ft/pkg

500 × 1.10 (waste) ÷ 90 sq ft per package = 6.1 packages.

Buy 7 packages

Frequently asked questions

How many bags of blown-in insulation do I need for 1,000 sq ft?
It depends on the R-value: about 16 bags for R-30, 20 for R-38 and 26 for R-49 with Owens Corning AttiCat (28.5 lb bags). Cellulose and other fiberglass brands publish their own charts with different rates. Always match the calculator to your bag's chart.
What R-value should my attic have?
US DOE guidance by climate zone runs from R-30 in the warmest zones to R-49–R-60 in cold ones; R-38 is the most common target for existing homes. Your local energy code sets the requirement for new work.
What size batts fit 16 in on-center studs?
15 in wide batts. They're made slightly narrower than the spacing so they friction-fit the 14.5 in cavity. For 24 in on-center framing, use 23 in batts. The package label states both the batt width and the sq ft it covers.
Can I blow new insulation over old insulation?
Usually yes. Blown fiberglass or cellulose can go over existing attic insulation as long as it's dry and not mouldy, and the added bags count toward the total R-value just as the chart says.
Does the calculator include the rental blower?
No, but note that home centers commonly loan the blower free with a minimum bag purchase (often 10+ bags), which is worth asking about when you buy.

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Estimate disclaimer: results are planning estimates based on standard manufacturer yields and coverage figures. Site conditions, product choice and installation method change actual quantities. Verify before you buy, and follow local building code for structural work. This tool does not replace professional engineering advice.

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